1) Field of the Invention
The invention relates to a method of controlling quality of pulp produced by mechanical defibering and by screening the pulp thereby obtained to provide at least two fractions, the accept that has passed the screening phase being carried forward for later use and the reject that has not passed the screening phase being led out of the screening phase.
2) Description of Related Art
In modern fiber processes of paper and board manufacture, the formed pulp is screened under pressure to keep the quality of the accepted pulp, i.e. accept, uniform. This may be carried out by controlling the amount of mass, i.e. the level of the mass surface, in the feeder or accept containers in the screening. Other alternatives include adjustments based on screening pressure and mass flow. In principle, these methods only control the capacity of the screening which is not, as such, in any way directly proportional to the quality of the screened pulp. Another way to control the screening such that the quality of the accepted pulp is also maintained as uniform as possible, irrespective of capacity variations, is based on adjusting the values of the flow-to-reject ratio and the feed consistency of the pulp supplied to the screening.
Although the adjustments used in prior art process control methods may be applied in standard conditions, they cannot be used for controlling the screening process in exceptional circumstances, for example in grade changes when the freeness value of the accept is to be changed or when the screening process is started up/shut down. Consequently, the quality of the pulp to be supplied to the screening process varies significantly, thereby affecting the further processes and the quality of the fiber web made of the pulp. The variations may be considerable, and the control of the screening process is substantially dependent on the process quality measurements. The prior art control parameters, such as the mass-to-reject ratio between the reject and the supplied pulp, are not sufficient to properly control changes in the quality of the accept. Even though there are ways to change the quality of the accept, the magnitude of the change cannot be predicted prior to the change. Consequently, the changes must always be followed by laboratory tests on the quality of the accept, such as the freeness value, fiber length distribution and fiber flexibility.